Field of the Technology
The disclosure relates to the field of portable broadband diffuse optical spectroscopic imaging device for noninvasive tissue characterization.
Description of the Prior Art
There are many prior art frequency-domain instruments that have been described in the literature (see Chance, B., Cope, M., Gratton, E., Ramanujam, N. & Tromberg, B. 1998 Phase measurement of light absorption and scatter in human tissue. Review of Scientific Instruments 69, 3457-81.) However, none of them use multi-frequency frequency-domain photon migration. None of them do this in a modular or networkable platform that allows for many smaller instruments to be linked together in lieu of a single larger instrument. None of them combine techniques to provide full broadband information content.
Frequency domain photon migration (FDPM) is an established technique for measuring tissue optical properties, i.e., absorption, μa, and reduced scattering, μa′. The FDPM technique uses light sources modulated at tens to hundreds of MHz to measure both the amplitude and the phase shift of multiply scattered light. Computational models are used to calculate absolute absorption and scattering tissue optical properties from these phase and amplitude measurements. Within the near-infrared infrared (NIR) spectral range (650 to 1000 nm) where FDPM is routinely employed, the tissue concentrations of oxygenated (ctO2Hb) and deoxygenated hemoglobins (ctHHb), water (ctH2O), bulk lipid, and other tissue constituents, both endogenous and exogenous, may be calculated from tissue absorption spectra. FDPM techniques can separate the effects of absorption from scattering using as little as a single spatial location, which is important because NIR light is strongly multiply scattered by tissues.
Technical challenges and practical limitations have prevented the widespread use of FDPM. In contrast to the complexity of FDPM, steady-state tissue spectroscopy methods are common because off-the-shelf systems are easily assembled from commercial vendors. Substantially fewer commercial FDPM devices are available, although at high cost. FDPM techniques are further complicated by introducing broadband FDPM methods, where the light sources are modulated over a range of frequencies. Increasing modulation bandwidth improves recovered optical property accuracy in both spectroscopy and imaging. A broadband FDPM instrument used in combination with a steady-state spectroscopy instrument to cover the entire MR spectral range has been involved in several pilot clinical studies. See for example, Quantitative Broadband Absorption and Scattering Spectroscopy in Turbid Media by Combined Frequency-Domain and Steady State Methodologies, U.S. Pat. No. 7,428,434, incorporated herein by reference. One embodiment of this instrument is the laser breast scanner (LBS) which represents a fairly standardized instrument platform. The core of the FDPM component of this instrument consists of a conventional network analyzer to generate and detect modulated radio frequency (RF) currents. For each laser diode (currently six), the network analyzer generates RF frequencies in series from 50 MHz to 1 GHz at 15-dBm electrical power. Laser diodes are DC-biased using a separate current source. An avalanche photo diode (APD) detects the diffuse reflectance from the tissue, sends the RF electronic signal back to the network analyzer, and compares this signal to a reference. The network analyzer measures the reflectance attenuation and phase shift between transmitter and receiver as functions of modulation frequency.
However, this network-analyzer-based FDPM instrument suffers from a number of technical limitations. First, the instrument is constructed using general-purpose commercial electronics and is therefore both expensive ($60 k total) and large in size. Second, the current FDPM instrument has run up against temporal performance limits. Sweeping 401 modulation frequencies takes approximately 1 s, but most of this time is wasted due to communication delays. Third, the high cost and large size of the instrument impedes expansion into multichannel imaging devices and increases barriers to access.